Not Applicable.
The present invention relates generally to marine riser systems and more particularly to a riser system having a small diameter riser that can be shifted to one side to allow access into and out of a well with large diameter casing, casing hangers and seal assemblies. Still more particularly, the present invention relates to a shiftable riser connection that continuously provides fluid communication between the well and the surface throughout the shifting operation and while accessing the large diameter borehole below the small diameter riser.
Drilling operations for the recovery of offshore deposits of crude oil and natural gas are taking place in deeper and deeper waters. Drilling operations in deeper waters are typically carried out from floating vessels rather than from stationary platforms resting on the ocean floor and commonly used in shallow water. According to conventional procedures, a drilling vessel is dynamically stationed, or moored, above a well site on the ocean floor. After a wellhead has been established, a blowout preventer (BOP) stack is mounted on the well head to control the pressure at the surface.
Subsea wells are typically drilled with multiple boreholes having decreasing diameters as the wellbore extends deeper into the earth. Each borehole is lined with a casing string that extends into the borehole from a wellhead and is cemented within the borehole. The drilling, casing installation and cementing is performed through one or more risers that extend from the wellhead to the surface, such as to a floating drilling vessel.
A riser pipe extends from the floating vessel to the wellhead equipment on the ocean floor to conduct downhole operations. The riser is attached to the wellhead equipment and is supported in tension at or near the water surface so as to prevent its collapse. In drilling the borehole for the well, a drill string is passed from the floating vessel down through the riser and wellhead equipment and into the borehole.
By way of example, a 21 inch riser usually extends from a blowout preventer (BOP) stack mounted on the wellhead on the sea floor to the drilling platform on the drilling vessel at the surface. Typically, the BOP stack has a 18-xc2xe inch nominal bore and is commonly used for rilling operations in almost any water depth from a floating vessel. The 21 inch riser typically has an outside diameter (OD) of nominally 21 inches and an inside diameter (ID) of nominally 19 inches. Thus operations are conducted through the 19 inch ID of the 21 inch riser and the bore of the BOP stack.
Generally the largest casing string installed in the wellbore is 16 inch OD casing and then after drilling the next borehole, a 13-xe2x85x9c inch OD casing is installed. Typically the next casing string to be installed is a 10-xc2xe inch OD casing or alternatively a 9-⅝ inch OD casing. The next casing string to be run is typically either a 7 or 7-⅝ inch OD casing.
When the casing extends to a depth where it will encounter substantial downhole pressures, the casing string must be run into the wellbore under well controlled conditions, i.e. through a drilling riser and BOP stack. The 13-xe2x85x9c inch casing typically reaches such depths and requires well control. Thus, the BOP stack must be large enough to accommodate the new casing string, such as a 13-xe2x85x9c inch casing, that is to be installed under well control. A 16 inch riser will accommodate casing, casing hangers and well tools having an OD of up to 13-xc2xd inches and thus a 16 inch riser will allow the passage of a 10-xc2xe inch OD casing and smaller. Neither a 16 or 13-xe2x85x9c inch casing string will pass through a 16 inch OD riser so a 21 inch riser may be required.
Wells are being drilled in deeper water, such as to depths of 10,000 feet, causing difficulties in using 21 inch risers. Because of the current drag forces and the weight of a 21 inch riser which is several thousand feet long and full of drilling mud, the large diameter riser becomes very unwieldy particularly in an ocean environment. The riser is maintained in tension from the floating drilling vessel and thus where a large diameter riser is several thousand feet long, the amount of tension that must be applied to the riser requires a very high tension force at the top of the riser on the vessel. This necessitates that the riser have increased strength to handle the increased tension thereby requiring that the thickness of the wall of the riser be increased which in turn increases the weight of the riser. The more weight that is required, the greater the tension that is required. Thus, the problem becomes greater as the length and size of the riser increases.
The floating drilling vessel must accommodate the riser required for downhole operations. Thus, the vessel must be specially equipped to handle large diameter risers and their associated large tension loads in deep water.
The drilling operation must be conducted through a riser which is large enough to accommodate the drill bit, the casing hangers, the seal assemblies and also provide an annulus around the new casing which is large enough to set and cement the casing. Typically, the drill pipe is 5 inch or 5-xc2xd inch OD pipe with the larger 5-xc2xd inch OD drill pipe typically being used in deeper water. Although typically the first bit into the well is a 17-xc2xd inch bit, an expanding bit, such as an underreamer, hole opener, or bi-center bit, maybe used where the bit has a smaller OD to pass through a small diameter riser. Once in the borehole, the bit will drill a larger diameter borehole.
Drilling mud is circulated down through the drill string and returned to the vessel through the annulus formed between the riser and the drill pipe. It is necessary for the 21 inch riser, extending several thousand feet, to handle all of the drilling mud needed for drilling the boreholes. Because of the difference in density between the drilling mud and sea water, the large pressure created by the fluid column in the large diameter riser must be contained within the riser. The column of drilling mud can be approximately twice as heavy as sea water such that for every foot of depth, there is about one-half psi of mud gradient weight whereby at a depth of 10,000 feet, there could be 5,000 psi inside the large diameter riser relative to the sea water around the riser.
The drilling fluids in the riser also form a fluid column placing a hydrostatic head on the well for well control purposes. Well control is established by maintaining the density of the drilling fluid, and thus the hydrostatic pressure exerted on the subsurface formations, at a level that is sufficient to prevent the production fluids under pressure in the formation from overcoming the hydrostatic head. If the hydrostatic head on the well is insufficient, the pressurized gas and other formation fluids may exceed the hydrostatic head leading to a blowout, sometimes resulting in damage to property, the pollution of the ocean and loss of life.
On the other hand, if the hydrostatic head is too great, the pressure may force drilling fluids into the formation causing the loss of drilling fluids into the formation or a reduction or lost in production. If too much drilling fluid is lost into the formation and the level of drilling fluid drops in the riser, the hydrostatic head can decrease below the pressure of the formation and cause a blowout. Furthermore, the hydrostatic head may increase to an amount so as to fracture the formation resulting in increased lost circulation.
According to conventional practice, choke and kill lines typically extend from the drilling vessel to the wellhead to provide fluid communication for well control and circulation. The choke line is in fluid communication with the borehole at the wellhead and bypasses the riser to vent gases or other formation fluids directly to the surface. According to conventional practice, a surface-mounted choke valve is connected to the terminal end of the choke conduit line. The downhole back pressure can be maintained substantially in equilibrium with the hydrostatic pressure of the column of drilling fluid in the riser annulus by adjusting the discharge rate through the choke valve.
The kill line is primarily used to control the density of the drilling mud. One method of controlling the density of the drilling mud is by the injection of relatively lighter drilling fluid through the kill line into the bottom of the riser to decrease the density of the drilling mud in the riser. On the other hand, if it is desired to increase mud density in the riser, a heavier drilling mud is injected through the kill line.
In addition to the choke and kill lines, a well may be provided with a booster line, through which additional mud can be pumped to a desired location so as to increase fluid velocity above that point and thereby improve the conveyance of drill cuttings to the surface. The booster line can also be used to modify the density of the mud in the annulus. By pumping lighter or heavier mud through the booster line, the average mud density above the booster connection point can be varied. References in the discussion below to choke, kill, and booster lines will be understood to include booster lines where desired. While the choke, kill, and booster lines provide pressure control means to supplement the hydrostatic control resulting from the fluid column in the riser, the riser itself provides the primary fluid conduit to the surface.
In deep water, however, the riser is the source of several disadvantages. Because the length of the riser must approximate the depth of the water, deep-water risers are expensive and quite heavy. The drilling vessel must support the riser in tension in order to keep the riser from buckling under its own weight. The riser is subject to lateral forces from currents in the water. In addition, the volume of drilling mud necessary to fill a deep-water riser is substantial. For a 10,000 ft depth application, a 21 inch riser can require over 6000 additional gallons (over 70% more) of mud to fill than a 16 inch riser. The expense of preparing and handling the large volume of drilling mud increases the cost of the well.
If it were possible to reduce the size of the riser, the riser would be lighter and less expensive, and would be subjected to smaller current loads. The expense associated with the volume of drilling mud required to fill the riser would be correspondingly reduced. Furthermore, the reduction in the size of the riser would subsequently reduce the need to increase the drilling fluid velocity to effectively lift cuttings away from the well. However, because the riser must be large enough to allow the passage therethrough of various large diameter casing and well tools that must be passed into the wellbore, it has heretofore been impossible to utilize a riser having an inside diameter smaller than the outside diameter of these large diameter objects. Hence it is desired to provide a small diameter riser system that allows hydraulic communication with and control of a deep-water well, while simultaneously allowing access to the well by large diameter equipment. Thus, using a small diameter, riser throughout the downhole drilling operation would provide many advantages.
Other objects and advantages of the invention will appear from the following description.
The riser system of the present invention utilizes a small diameter riser while continuously providing hydraulic communication between the wellhead and surface maintaining control of a deep-water well and simultaneously allowing access to the well by large diameter equipment. The riser system allows large diameter casing and other equipment to be placed in the well even though the riser that provides fluid communication with the well during drilling has a smaller inside diameter than that of the outside diameter of the equipment. The riser system further allows access to the well by the large diameter equipment without requiring that the small diameter riser be emptied of drilling mud, that the well be filled with extra-dense mud, or that fluid communication with the well be suspended.
More particularly, the riser system provides a partial disconnect between the riser and the well so that the riser can be closed to retain the column of drilling fluid therein while still allowing fluid communication with the well. According to one preferred embodiment, when it is desirable to provide large diameter access to the well, the small diameter riser is disconnected from a large diameter riser joint connected to the BOP stack and well. Choke, kill and booster lines remain connected and open to fluid communication between the flowbore of the small diameter riser and the well. One or more flexible connections are provided to allow the disconnected riser to shift laterally away from the large diameter riser joint. According to a preferred embodiment, hydraulic rams push against the riser joint and choke, kill and booster lines to shift the small diameter riser apart, thus providing room for access to the large diameter riser joint for installing large diameter equipment such as casing into the wellbore. The same hydraulic action preferably causes a guide to align the large diameter equipment with the top of the large diameter riser joint to facilitate passage of the large diameter equipment into the wellbore.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.